Cellulose which is a main component of pulp prepared by purification of wood or cotton linter cannot be molecularly dispersed even if it is dispersed in water owing to hydrogen bonding between hydroxyl groups existing in its molecule. This is presumed to occur because cellulose is a high molecule and its crystalline portion formed by the hydrogen bonding disturbs hydration, whereby the cellulose is insoluble in water. The molecular dispersion in water is improved by derivatizing the hydroxyl group portion of the cellulose derivative by an ester or ether bond to break the hydrogen bonding.
Cellulose is soluble only in a special solvent such as a copper ammonia solution, carbon disulfide with caustic soda, or dimethylsulfoxide/paraformaldehyde. When the cellulose is derivatized, addition of a derivatization reagent to the cellulose in the molecularly dispersed state after dissolving in a solvent is not commonly done in the industry because of difficulty in recovery or reuse of the solvent. Particularly when a cellulose derivative having water solubility is prepared, cellulose in the solid form is dispersed in an alkali solution for activation of the reaction or is impregnated in an alkali, and is then brought into contact with a reaction reagent to derivatize the cellulose. The reaction reagent does not freely cause reaction with the three hydroxyl groups existing around pyranose rings which are repeating structures of cellulose. For example, all of the hydroxyl groups around chained pyranose rings have no reaction, or only one of these hydroxyl groups has reaction. Consequently, portions at which an intramolecular hydrogen bond cannot be broken may exist at intervals of several tens or several hundreds of μm.
When a cellulose derivative having a portion in which the reaction has not progressed sufficiently is dissolved in water, even if the reaction reagent clings to the cellulose derivative in form of replacement or addition, a portion in which a hydrogen bond between hydroxyl groups is broken is dissolved in water, while a portion, in which the reaction has not progressed sufficiently and an amount of replacement of or addition to the hydroxyl group of the cellulose by the reaction reagent is low, is insoluble and floating in water.
Cellulose derivatives are used for various purposes. When they are used for film formation, an undissolved floating portion sometimes impairs the quality of the resulting film. For example, conventional extruded ceramic products are manufactured as follows. Main ceramic materials are mixed, calcined and ground to produce a nonplastic ceramics powder. Added thereto are a cellulose derivative as a binder for imparting it with plasticity, water for dissolving the binder therein, an optional plasticizer and an optional lubricant. The resulting mixture is kneaded (mixing while tearing into pieces using a roll mill, a continuous kneader or the like) to produce a argil. Alternately, the binder dissolved in water is kneaded with the ceramic powder sufficiently to produce an argil. The argil is extruded and then calcined to produce a green body. Particularly when a green body such as a ceramic sheet for electronic materials which need high quality is prepared, a water-soluble cellulose ether such as methoxy-containing methyl cellulose has been used widely as the binder to enhance water retention and shape retention of the argil and to impart it with plasticity suited for extrusion even by the addition of a small amount.
In order to obtain cellulose ether having a desired polymerization degree, raw material cellulose having a polymerization degree within a predetermined range is selected and subjected to etherification reaction. Alternatively, cellulose (pulp) having a proper polymerization degree is selected and dipped in an alkali solution to yield an alkali cellulose. The resulting alkali cellulose is allowed to stand (be aged) in the air under appropriate conditions (temperature, time and the like) and then subjected to etherification reaction. Particularly, cellulose ether to be used as a material for extrusion of ceramics preferably has a polymerization degree corresponding to the viscosity at 20° C. of from 25 to 200,000 mPa·s in a 2% by weight aqueous solution. When it has a polymerization degree falling within this range, it can be extruded easily because of low stickiness during extrusion and the extruded product does not crack during drying.
Such cellulose ether is obtained by ether substitution of natural cellulose, which is essentially insoluble in water, in order to give it water solubility. The substitution reaction is a solid/liquid reaction between cellulose and an etherifying agent as described above so that the reaction tends to be heterogeneous in the industrial-scale production and a low-substituted portion is not easily soluble in water. When such cellulose ether is employed for extrusion of ceramics, an undissolved portion of the cellulose ether remains in the extruded product and the subsequent calcinations generates pores of about several μm.
Existence of such pores may be an obstacle for the preparation of an IC substrate in which conductive and insulator layers having thickness of about 10 to 20 μm are printed on a sintered substrate. These pores also deteriorate withstanding voltage properties of ceramics sintered body for a dielectric such as a capacitor. According to Japanese Patent Provisional Publication No. 01-111770, there is a strong demand for the development of ceramic materials having no such defects.
In addition, cellulose ether having a low polymerization degree or cellulose ester formed by esterification of the cellulose ether and made enteric such as hydroxypropylmethyl cellulose acetate succinate, hydroxypropylmethyl cellulose phthalate or cellulose acetate phthalate is dissolved in a solution of alkali such as ammonia and the resulting solution is spray-coated onto tablets or granules containing a drug and dried. When an undissolved portion exists, a spray gun is clogged therewith or a coated part contains defects, leading to problems that contrary to the intended purpose of the coating, bitterness of the drug is tasted or the drug is released in the stomach from the defective portion because a function of masking the bitterness of the drug or releasing the drug not in the stomach but in the intestine is not fulfilled.